1. Field of the Invention
The present invention relates to an image forming apparatus and a toner supply method.
2. Description of the Related Art
In the related art, a single-component developing apparatus is known. In the single-component developing apparatus, developing is carried out in such a manner that a developing roller, which is a toner carrying member, carries toner that is developer of a non-magnetic or magnetic single component, and the toner on the developing roller is supplied to a latent image on a photosensitive member, in a developing area in which the photosensitive member that is a latent image carrying member and the developing roller face one another.
In the single-component developing apparatus, the developing apparatus is replaced when the toner in the developing apparatus has run out. Therefore, the developing roller for which a replacement time has not been reached yet and thus usage of which can be continued is also replaced. Thus, a resource may be wasted. In a case where the developing apparatus is configured such that a time at which the toner in the developing apparatus runs out and the replacement time of the developing roller are the same as one another, it may be necessary to ensure a space for holding a great amount of toner in the developing apparatus, and thus, the developing apparatus may be increased in size.
Japanese Patent No. 4026977 (patent document 1) describes an image forming apparatus in which a toner container that holds toner is provided separate from a developing apparatus, and a supplying part supplies the toner of the toner container to the developing apparatus. Thereby, in a case where the toner in the toner container runs out, only the toner container is to be replaced, and thus, a developing roller for which usage can be continued is not to be replaced. Further, because an amount of the new toner held by the toner container may be determined without regard to a replacement time of the developing roller, it is possible to reduce a capacity of the toner container to be small, and thus, it is possible to prevent the image forming apparatus from being increased in size.
Further, according to the image forming apparatus of the patent document 1, the toner of the toner container is supplied to the developing apparatus when the toner amount in the developing apparatus becomes less than a lower limit value. Therefore, after the toner is thus supplied to the developing apparatus, the toner having remained in the developing apparatus for a long period of time without being used for developing and the toner newly supplied from the toner container are mixed together.
Japanese Laid-Open Patent Application No. 2009-75244 (patent document 2) describes the following image forming apparatus for preventing fog caused as a result of new toner being supplied to a developing apparatus in which old deteriorated toner remains. The term “fog” means a phenomenon that an image area that is to be a blank has a density increased as a result of toner adhering thereto through a developing process. That is, when a toner amount in the developing apparatus becomes less than a lower limit value, such control is carried out that, before toner is supplied to the developing apparatus, the toner remaining in the developing apparatus is ejected toward an image carrying member. Thereby, the old toner remaining in the developing apparatus is ejected to the image carrying member, and, in a condition in which the developing apparatus has thus become approximately empty of toner, new toner is supplied from a toner container. Therefore, almost all of the toner in the developing apparatus becomes the new toner after the new toner is thus supplied, and thus, it is possible to prevent fog after the new toner is supplied.
A reason why fog occurs when deteriorated old toner and new toner are mixed will now be described concretely.
Old toner remaining in a developing apparatus has suffered stress for a long period of time due to such as stirring. As a result, an external additive that is added externally to surfaces of toner particles for controlling flowability and an electrification property may have been removed or may have been embedded in the particles. Thereby, the toner may not be easily electrified frictionally to, for example, negative polarity that is normal electrification polarity of the toner. On the other hand, new toner supplied to the developing apparatus is not deteriorated and thus, is easily electrified frictionally to the negative polarity. Therefore, when the new toner that is easily electrified to the negative polarity and the old toner that is not easily electrified to the negative polarity are rubbed together, charge separation occurs, and electrons in the old toner move to the new toner. As a result, an electrification amount of the new toner to the negative polarity may increase, an electrification amount of the old toner to the negative polarity may decrease, or the old toner may be electrified to positive polarity. As a result, the toner electrification distribution becomes broad, and also, such a distribution may occur in which two peaks, i.e., an area in which the electrification amount to the negative polarity is large and an area in which the electrification amount is approximately zero, exist. Thus, after the new toner is supplied, the deteriorated toner may become weak electrified toner, or reverse electrified toner. Therefore, in an image forming process after the new toner is supplied, the above-mentioned deteriorated old toner may adhere to an area (other than a latent image area) on a photosensitive member that is an image carrying member for which area no toner is desired to be placed. As a result, fog increases in comparison to a case before the new toner is supplied.
However, there may be case where, for example, a toner consumption rate per a unit period of time in a developing apparatus is high, remaining toner in the developing apparatus becomes equal to or less than a predetermined value within a short period of time, the toner in the developing apparatus has suffered not much stress, and thus, deterioration of the old toner remaining in the developing apparatus is minor. In such a case where deterioration of the old toner remaining in the developing apparatus is minor, the toner has sufficient electrification capability, so that charge separation hardly occurs even when new toner is supplied and the old toner and the new toner are rubbed together. Therefore, in such a case where deterioration of the old toner remaining in the developing apparatus is minor, a toner electrification distribution in the developing apparatus after the new toner is supplied can be maintained as a sharp distribution having a peak of a predetermined electrification amount of the negative polarity. As a result, it is possible to obtain an image in which fog is prevented after the new toner is supplied.
However, according to the above-mentioned patent document 2, although deterioration of old toner remaining in the developing apparatus is minor, and thus, the toner has sufficient electrification capability, the toner is ejected to the image carrying member, and thus, is discarded. As a result, the toner may be wasted.